bramber said..
You provide a beacon of hope! What is the best age to start teaching them? My eldest is 2.5.
The answer really is when the kid is ready. You know it when they start having fun and getting stoked about all these very little achievements they do. Kiting can be intimidating for kids ... fear will drop the fun factor. Most kids don't think like adults, they do not understand that investing time in things they don't like or don't want to do (cold water, walking on sea weed, etc.) will have long term benefits. If it is not funny or if it is scary, they will just not want to do it. So they need to have fun and be comfortable during every step of their progression.
At 2.5 years, they can start playing with a normal/training kite. See how he likes it. If you can see there is a lot of interest, try to encourage him paying with it from time to time. If not, he is not ready yet.
All you need to do is to make equipment available (trainer kite), give him opportunities to use the equipment, encourage him when he is doing good, support him when he is not doing good, push a little bit but not too much.